After water reaches its boiling point, it becomes moist steam that is a mixture of water vapor gas (the gas phase part) and water droplets (the liquid phase part). Here the weight ratio of the water vapor gas relative to the moist steam is termed the “dryness fraction.” For example, if water vapor gas and water droplets exist at half each, the dryness fraction would be 0.5. Moreover, when there are no water droplets but instead there is only water vapor gas, then the dryness fraction would be 1.0. From the perspective of efficiency of use of the sensible heat and latent heat within the moist steam in heat exchanging equipment, and the like, from the perspective of preventing corrosion of turbine blades in steam turbines, and so forth, it is desirable that the dryness fraction of the moist steam be brought to near 1.0. Because of this, a variety of methods have been proposed whereby to measure the moist steam.
For example, Japanese Unexamined Patent Application Publication H8-312908 (“JP '908”) discloses a technology for calculating the dryness fraction by calculating the saturated hydraulic enthalpy and the saturated steam enthalpy using a saturated steam table based on the dry steam flow rates and pressures before and after a pressure regulating valve, taking advantage of the fact that there is no change in total enthalpy across a pressure regulating valve that is disposed in a pipe. Moreover, Japanese Unexamined Patent Application Publication 2000-121616 (“JP '616”) discloses a technology for calculating the dryness fraction based on acoustic velocity through the provision of a branch flow pipe in a pipe and the provision of ultrasonic receiver within the branch flow pipe.
However, in the technology disclosed in JP '908, high accuracy dryness fraction measurements are difficult, considering the complexity of the structure, and the increasingly large compounding tolerances due to the need to provide a plurality of flow rate sensors and pressure sensors.
The technology disclosed in JP '616 requires the provision of a branch pipe, requiring great care in the installation. Moreover, it requires expensive ultrasound receiving equipment able to endure the high temperature and high pressure environment. Moreover, while acoustic velocity and dryness fraction are correlated, pressure also has an effect on the dryness fraction, and thus a separate pressure sensor is required as well.
Given this, one object of the present invention is the provision of a dryness fraction measuring device and dryness fraction measuring method whereby the dryness fraction can be measured accurately and easily.